Method and composition for increasing the corrosion resistance of phosphatetype chemical conversion coatings on metal surfaces



METHOD AND CGMPQSITEON FOR INtIREASiNG TIE CORRDSEON RESESTANCE F PHGSRHATE- TYPE CHEMECAL CGNVERSKON COATEWGS 0N METAL SURFACES George Schneider, Trevose, Pa., assignor to Amchem Products, inc, Ambler, Pa, a corporation of Deiaware No Drawing. Filed Sept. 7, 1960, Ser. No. 54,359

Claims. (Cl. 148-616) This invention relates to the art of treating metal surfaces and, more particularly, is directed to a process for improving the characteristics of surfaces which have already had the benefit of a previously applied chemical conversion coating regardless of whether or not they are to be given a subsequent siccative coating such for example as a coating of a sanitary enamel, a lacquer, a paint or the like.

Before stating the objects of the invention or describing it in detail, it should be recalled that it is now well known that increased corrosion resistance can be imparted to metal surfaces by forming on the surface a chemical conversion coating-such coatings also serving as very satisfactory pre-paint bonding mediums. Some of the most common conversion coatings employed in the 'art consist of those which are based on the use of solutins of phosphates and/or chromates as the anion filmforming ingredients. The phosphate type coatings are represented by the so-called heavy metal coating phosphates, typical solutions of which are described in US. Patents 2,813,812; 2,820,731, and 2,835,617. They are also typified by the so-called non-coating phosphates such as those which are exemplified in US. Patents 2,252,874; 2,886,477 and 2,891,884. The chromate type conversion coatings generally familiar to the art are produced by the use of aqueous acid solutions containing fluorine and hexavalent chromium ions either with or without phosphate ions and typical examples of these are found in US. Patents 2,438,877; 2,796,370 and 2,851,385.

Keeping the foregoing in mind, the principal object of the present invention resides in the provision of a process for improving the usefulness and especially the corrosion resistance of steel, aluminum or zinc upon which a previously applied chemical conversion coating has been applied. An associated object is to provide a process for the production of improved corrosion resistant films on steel surfaces which are especially suitable as a base for the subsequent application of sanitary coatings such as used on the interior surfaces of containers for the packaging of foods.

Additional objects involve the provision of a film on previously coated metal surfaces which film is highly water-insoluble and also the provision of a process for applying a film of the character described which is cheaper to apply by a process which is less trouble to operate.

Heretofore it has been proposed to treat clean surfaces of steel, aluminum or zinc with an aqueous acid solution containing, as its principal and essential ingredients, chromic and citric acids and that such treatment followed by a subsequent drying step will provide a film on the surface of the bare metal which imparts a high degree of corrosion protection, but insofar as I am aware, no one has suggested that solutions of this kind, namely those consisting essentially of a mixture of chromic and citric acids, can ;be employed as a rinse over previously applied chemical conversion coatings.

The present invention is based upon the discovery that solutions of the character described can be applied as a rinse to chemical conversion coatings as a means for greatly enhancing their useful qualities and especially the corrosion resistance thereof provided the solution is maintained within a relatively narrow range of solution con- 3,079,288 Patented Feb. 26, 1963 -ice cen-tration. This range has been found to comprise chromic-citric acid solutions which contain from 1.7 to 3.4% of total initial solids with from to being CrO This limited range of total initial solids within the proportions indicated provides a rinsing formulation which yields very marked improvement in the characteristics and corrosion resistance of previously applied chemical conversion coatings.

Employment of less than 50% of chromic acid in the total initial solids content of such a rinsing solution yields results which are poor in corrosion resistance while the use of more than 60% of chromic acid in the total initial solids results in soluble films of hexavalent chromium and poor corrosion protection of the treated surfaces.

In preparing a coating or rinsing solution according to this invention it is important that chromic and citric acids per so be employed. Use of alkali metal or ammonium salts of these acids is to be avoided since it has been found that their presence in the films reduces the water repellancy and'the corrosion resistance thereof. However, the solutions of this invention are capable of tolerating small amounts of contaminating ions such, for example, as sulfate, phosphate, fluoride and ferric ions with no noticeable deleterious effects on the corrosion resistance of the treated surfaces.

The pH of the rinsing baths of the present invention is equally important and should be maintained at a level below about 4.5 because it'ha-s been found that pH values above this point result in the production of films which tend to have increasing water-solubility. Generally speaking, the solutions of this invention when prepared in accordance with this disclosure have been found to have a pH of approximately 3.5 so that, in the absence of alkali metal, ammonium or other cations, the baths automatically have a pH which lies below the maximum permissible level of about 4.5. If it should rise above this point due, for example, to drag-in of alkali cleaners which may have been used in preparing the surface for the treating operation, it can be adjusted by adding a minimal amount of acid such as nitric, sulfuric or phosphoric or the inclusion of additional citric acid.

The temperature at which the solutions of this invention may be employed ranges from average living or room temperatures up to about 140 F. However, it has been found that temperatures lying within the range of about to F. produce the best results and for this reason I prefer to operate within this range, especially since this preferred range offers greater economy of operation and serves to extend the life of the bath.

The corrosion resistant films resulting from the present invention mav be obtained by applying the solutions by spraying, rolling or dipping or by any other convenient manner familiar to the art. It is important that the surface to be treated be thoroughly wetted with the treating solutions of this invention, and in this connection the time of treatment may be varied from as little as two seconds to as much as five minutes. In practice, I have found that entirely satisfactory results may be obtained by using a contact time of from about 5 to about 30- seconds, although the broader range may be employed if desired.

The surface must be dried after treatment with the solutions of this invention and this may be accomplished merely by allowing the treated surfaces to air dry at room temperatures. However, this technique necessitates a curing cycle of about 4 hours. More rapid drying is generally preferred in industrial installations and it has been found that completely acceptable results in such installations can be secured by employing a 30 second to a 2 minute drying cycle at from about 250 to 300 F. Where a final-siccative finish is to be applied to the rinsed Example I Chromic acid 15 grams. Citric acid 10 grams. Water, to m lliter.

The solution of Example I had a pH of 3.6 and was sprayed over steel panels which had previously been treated with a commercial, cold, iron pyrophosphate coating bath, sold commercially under the proprietary name of Granodine 1 663. A 5 second spray cycle at room temperature was employed. Additional steel panels which also had been coated by the same commercial pyrophosphate bath were rinsed with standard chromic and/or phosphoric acid solutions according to the practices of the art. All of these panels were then finished with a Gilsonite baking enamel and subjected to a standard 5% salt spray test at 95 F. for 336 hours. The following table summarizes the results obtained.

Panel appearance after Acid rinse used: 336 hours testing The solution of Example II had a pH of 3.5 and was sprayed over zinc phosphate coated steel panels using a 5 second spraying cycle at room temperature. Other similar zinc phosphate coated steel panels were subjected to the rinsing action of the familiar chromic acid solution. All of the treated and rinsed panels were then subsequently painted with a baking enamel and subjected to the 5% salt spray test for 336 hours. Comparison of the results showed a decided improvement in the corrosion resistance of the chromic-citric acid rinsed panels over that exhibited by the panels which had been rinsed with chromic acid alone.

Example 111 Chromic acid 8.5 grams. Citric acid 8.5 grams. Water, to make 1 liter.

Registered trademark or Amchem Products, Inc.

panels and fair corrosion resistance for the panels which had been rinsed in chromic acid alone.

Example IV Chromic acid 10.5 grams. Citric acid 9.5 grams. Water, to make 1 liter.

Steel panels which had been treated in a dilute aqueous coating solution of primary and secondary alkali metal phosphates were subjected to the rinsing action of the Example IV solution, using a 5 minute spraying cycle at room temperature. Additional steel panels which had been coated in the same alkali metal phosphate coating solution were rinsed with standard chromic and/or phosphoric acid solutions according to the practices of the art. All of these panels were then finished with a Gilsonite baking enamel and subjected to a standard 5% salt spray test at 95 F. for 336 hours. The following table summarizes the results obtained:

Panel appearance after Acid rinse used: 336 hours testing Chromic plus citric Excellent. Chromic Fair to good. Phosphoric plus chromic Fair. Phosphoric Poor.

I claim:

1. The method of increasing the corrosion resistance of metal surfaces which carry a phosphate-type chemical conversion coating which comprises treating the coated surface with a solution consisting essentially of chromic acid, citric acid and water and drying the treated surface, the amount of total initial solids (i.e., citric acid plus chromic acid) being from 1.7 to 3.4% weight/volume (i.e. grams per 100 mls.) of the treating solution with the concentration of citric acid being from 40 to 50% of the total initial solids of the solution and with the pH lying elow about 4.5.

2. The method of claim 1 wherein the temperature of the solution does not exceed about 140 F.

3. The method of claim 1 wherein the temperature of the solution lies between and F.

4. An aqueous solution for treating chemical conversion coatings on the surface of metal said solution consisting essentially of chromic acid, citric acid and water, the amount of citric acid plus chromic acid being from 1.7 to 3.4% weight/volume (i.e. grams per mls.) with the concentration of citric acid being from 40 to 50% of the total, with the pH of the solution lying below about 4.5.

5. A solution according to claim 4 wherein the ingredients are present in the following proportions:

Chromic acid 15 grams. Citric acid 10 grams. Water, to make 1 liter.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF INCREASING THE CORROSION RESISTANCE OF METAL SURFACES WHICH CARRY A PHOSPHATE-TYPE CHEMICAL CONVERSION COATING WHICH COMPRISES TREATING THE COATED SURFACE WITH A SOLUTION CONSISTING ESSENTIALLY OF CHROMIC ACID, CITRIC ACID AND WATER AND DRYING THE TREATED SURFACE, THE AMOUNT OF TOTAL INITIAL SOLIDS (IE., CITRIC ACID PLUS CHROMIC ACID) BEING FROM 1.7 TO 3.4% WEIGHT/VOLUME (I.E. GRAMS PER 100 MLS.) OF THE TREATING SOLUTION WITH THE CONCENTRATION OF CITRIC ACID BEING FROM 40 TO 50% OF THE TOTAL INITIAL SOLIDS OF THE SOLUTION AND WITH THE PH LYING BELOW ABOUT 4.5.
 4. AN AQUEOUS SOLUTION FOR TREATING CHEMICAL CONVERSION COATINGS ON THE SURFACE OF METAL SAID SOLUTION CONSISTING ESSENTIALLY OF CHROMIC ACID, CITRIC ACID AND WATER, THE AMOUNT OF CITRIC ACID PLUS CHROMIC ACID BEING FROM 1.7 TO 3.4% WEIGHT/VOLUME (I.E. GRAMS PER 100 MLS.) WITH THE CONCENTRATION OF CITRIC ACID BEING FROM 40 TO 50% OF THE TOTAL, WITH THE PH OF THE SOLUTION LYING BELOW ABOUT 4.5 